Background: The COVID-19 pandemic has disrupted routine hospital services globally. This study estimated the total number of adult elective operations that would be cancelled worldwide during the 12 weeks of peak disruption due to COVID-19. Methods: A global expert-response study was conducted to elicit projections for the proportion of elective surgery that would be cancelled or postponed during the 12 weeks of peak disruption. A Bayesian beta-regression model was used to estimate 12-week cancellation rates for 190 countries. Elective surgical case- mix data, stratified by specialty and indication (cancer versus benign surgery), was determined. This case-mix was applied to country-level surgical volumes. The 12-week cancellation rates were then applied to these figures to calculate total cancelled operations. Results: The best estimate was that 28, 404, 603 operations would be cancelled or postponed during the peak 12 weeks of disruption due to COVID-19 (2, 367, 050 operations per week). Most would be operations for benign disease (90.2%, 25, 638, 922/28, 404, 603). The overall 12-week cancellation rate would be 72.3%. Globally, 81.7% (25, 638, 921/31, 378, 062) of benign surgery, 37.7% (2, 324, 069/6, 162, 311) of cancer surgery, and 25.4% (441, 611/1, 735, 483) of elective Caesarean sections would be cancelled or postponed. If countries increase their normal surgical volume by 20% post-pandemic, it would take a median 45 weeks to clear the backlog of operations resulting from COVID-19 disruption. Conclusions A very large number of operations will be cancelled or postponed due to disruption caused by COVID-19. Governments should mitigate against this major burden on patients by developing recovery plans and implementing strategies to safely restore surgical activity.